The present invention relates to medical devices and pertains particularly to an improved rod fixation system for fractures in long bones.
Fractures in long bones in the human body can properly heal only if the two portions of the fractured bones are properly positioned and fixed relative to each other. Such fractures are frequently assisted in their mending by utilizing either an external or internal splint. Internal splints are preferred for most fractures and consist of an elongated intramedullary rod or nail inserted in a bore extending along the axis of the bone and secured by screws to tie the two parts of the fractured bone together until healing can occur. A major difficulty with prior art devices in the case of humerus is that non-union or delayed union frequently occurs. I have found that part of the reason is that the applied force is tensile force rather than compression required to force the two parts of the fractured bone together to enable mending.
In my prior patent, U.S. Pat. No. 5,480,402, entitled "Shoulder Compression Interlocking System", I disclosed an improved rod and interlocking system which improved the stabilization of bone fractures. However, that structure was unable to apply the necessary compressive forces discovered to be necessary.
In fracture healing, there are two basic requirements: osteoconduction and osteoinduction. Osteoconduction is a physical, mechanical requirement. Contact or continuity of bone ends is important for fracture healing. Osteoinduction is the consideration of biological biomechanical induction of bone healing and bone formation. Blood circulation, soft tissue preservation, Wolf's law are important considerations for osteoinduction.
Nonsurgical treatment with cast application stresses osteoinduction aspect in fracture healing. Surgical fixation, such as plate fixation stresses more of the osteoconduction aspect. In this method, a small gap in the fracture fixation can be very harmful with danger of nonunion developing. Successful healing of the callus tissue is affected by the stress it received (Perren). Stress is a function of the following factors: ##EQU1## .DELTA.l=motion in the gap l=width of the gap
Therefore, a small gap in internally fixed fracture imposes greater danger of developing nonunion than a nonoperative fracture with a larger gap. Under such circumstances, there is already less reliance on osteoinduction and the benefit of osteoconduction is compromised by micromotion significant in proportion to the size of the gap.
Intramedullary rod fixation without interlocking provides internal splint effect. It gives closer reduction to provide better anatomical alignment and osteoconduction effect. Weight bearing allows compression of fracture site narrowing gaps. The rod still allows transfer of Wolf's stress and osteoinduction. However, intramedullary rod system does not provide torsional stability. Therefore interlocking intramedullary fixation system was devised to compensate for torsional stability or to prevent excessive shortening when a comminuted fracture did not provide cortical stability.
Interlocking intramedullary rod systems are currently widely used. As a result, the interlocking system, though it may provide rotational stability, usually creates fixed gap at the fracture site. The gap bypasses the stress from the bone to the implant sometimes causing implant failure such as screw or rod breakage. Delayed union is the frequent result.
Interlocking rod fixation does not completely eliminate micro motion due to an oscillating or windshield wiper effect. To eliminate the windshield wiper effect, some manufacturers made two plane interlocking in the distal tibia. The two plane interlocking requires more soft tissue disturbance and increases risks of injury.
A more ideal system would be an interlocking rod with minimal micromotion but with compression or elimination of any gap. The benefits of my new invention in comparison can be summarized as follows:
1) Narrows the fracture gap to the point of contact providing osteoconduction. PA1 2) Eliminates micromotion. PA1 3) Provides dynamic compression at the fracture site, and therefore, osteoinduction through Wolf's stress. PA1 4) Shares the stress through the fractured long bone between the bone and the implant rather than having the implant take up the entire stress. As a result, implant failure such as rod or screw breakage will be minimized. PA1 5) Reduces motion at the distal interlocking (windshield wiper motion) by internal locking of the rod with this newly designed compression screw. Therefore, with one plane approach it achieves the benefit of two plane stability.
There is a need for an improved intramedullary system for providing compression to the fracture site to help bring about faster healing with less implant failure.
It is therefore desirable to have a long bone fixation device that is simple and more effective than present systems.